Conventional valves for gas vessels are little suited for situations wherein gas of the highest purity is required for manufacturing purposes such as, for example, in the manufacture of semiconductors. The utilization of the superpure gas does not permit any contamination by even the smallest quantity which could be present in the valve chamber or the gas-takeoff line of these valves. Furthermore, special superpure gases are incompatible with air and can be corrosive or toxic. For example, silane explodes when it comes into contact with residual air in the valve chamber. Toxic gases would endanger working personnel.
After interrupting or opening the gas-conducting system, the equipment can be first made inert when starting again by purging all output supply lines with a suitable gas via a separate purging circuit. However, the closure valve remains inadequately purged by this purging operation because it is not arranged in the direct purging flow; instead, the closure valve is accessible only through the gas-takeoff line because the gas supply is located at the inlet end. To purge blind spaces of the closure valve itself, separate manipulations are necessary such as interval purgings which are costly and the result is uncertain and, in addition, the gas-takeoff line must be separated from the remaining consumer lines. The impurities which remain do not then correspond to the high requirements which are imposed on the superpure gas atmosphere.
German Pat. No. 667,378 discloses a valve for closing high-pressure gas vessels while at the same time functioning as a venting valve for gas-takeoff lines. A closure bolt alternately opens and closes a high-pressure valve as well as a ventilating valve. When the high-pressure valve is open, the gas flows out from the vessel into the gas-takeoff lines to a consumer. If the vessel is to be closed, the closure bolt is pressed against the high-pressure valve while at the same time clearing the ventilating valve. The overpressure present in the valve chamber and in the gas-takeoff line can escape into the ambient via the opened ventilating valve.
This known valve is not suitable for superpure gases because for each interruption in operation and closure of the high-pressure valve, the valve chamber and the gas-takeoff line are brought into communication with the ambient via the ventilating valve so that a contamination is again present from the ambient. A further undesirable condition is present in that the gas supply is connected with the ambient during the transition phase during opening and closing because the high-pressure and ventilating valves are open at the same time.